Sound-absorbing materials are conventionally used for, for example, electric products, building wall materials, and vehicles. Particularly, for the purpose of preventing vehicles such as cars from generating exterior acceleration noise or exterior idling noise, specifications requiring that engines and transmissions be surrounded with acoustic shields are being adopted. Generally, in the case of cars, such acoustic shields need not only to have excellent sound absorbency but also to prevent the spread of fire to a driver seat in the event that a fire breaks out in an engine room due to a traffic accident, in view of securing safety. Accordingly, from the viewpoint of fire prevention, there has been a demand for a flame-retardant sound-absorbing material excellent in not only sound absorbency but also fire safety. In addition, it is also desired that such a flame-retardant sound-absorbing material should not produce a toxic gas when burned.
In addition to having sound absorbency and flame retardancy, it is desired that sound-absorbing materials for vehicles such as cars should be made of light and recyclable materials to achieve the weight reduction of cars and to promote recycled use of sound-absorbing materials of scrap cars. This is because promotion of recycled use of various parts of scrap cars to reduce the amount of industrial waste from scrap cars as much as possible is considered important for prevention of pollution.
For these reasons described above, light and flame-retardant non-woven fabrics are receiving attention as materials satisfying the above requirements. Generally, flame-retardant non-woven fabrics are manufactured by, for example, using flame-retardant fibers such as aramid fibers and polychlal fibers as main constituent synthetic fibers of non-woven fabrics, or using synthetic fibers in which a phosphoric acid-based flame retardant or a boric acid-based flame retardant is blended, or coating or impregnating sheet-like non-woven fabrics with a binder coating solution in which a flame retardant is dispersed.
For example, Japanese Patent Application Laid-open Nos. 62-43336 and 62-43337 disclose an interior material for vehicles manufactured by applying a vinyl chloride emulsion onto the surface of a non-woven fabric mat obtained by needle-punching a web comprised of 95 wt % of a polyester fiber, a polypropylene fiber, or a mixture thereof and 5 wt % of a rayon fiber, drying it to form a flame-retardant resin coating, and laminating a glass fiber mat on the resin-coated surface of the non-woven fabric mat to unite the glass fiber mat with the non-woven fabric mat. Such an interior material for vehicles is excellent in flame retardancy but poor in recyclability because the non-woven fabric mat is united with the glass fiber mat. Further, the interior material for vehicles has a problem in that there is a fear that the interior material produces dioxin when incinerated.
Further, Japanese Patent Application Laid-open No. 9-59857 discloses a flame-retardant non-woven fabric manufactured by laminating non-woven web layers of a flame-retardant staple fiber on both of the surfaces of non-woven web layers of a polyester fiber in such a manner that the amount of the non-woven web layers of the flame-retardant staple fiber becomes 50 wt % or more of the total amount of a resultant non-woven fabric, and intertwining the constituent fibers with each other between adjacent web layers. Japanese Patent Application Laid-open No. 2002-348766 discloses a flame-retardant sheet material manufactured by needle-punching a web obtained by blending a polyester fiber with a flame-retardant rayon fiber or modacrylic fiber (that is obtained by copolymerizing acrylonitrile with a vinyl chloride-based monomer as a flame retardant) and further stitch-bonding it. Japanese Patent Application Laid-open No. 2000-328418 discloses a halogen-free flame-retardant non-woven fabric manufactured by binding a fiber web containing a cellulose-based fiber, a polyvinyl alcohol-based fiber, and a phosphorus-based flame-retardant polyester fiber with an acrylic resin binder. These non-woven fabrics disclosed in the above patent documents are excellent in flame retardancy but poor in sound absorbency.
As an example of a flame-retardant sound-absorbing material, Japanese Patent Application Laid-open No. 2002-287767 discloses a sound-absorbing material for vehicles manufactured by coating and integrally molding a mat-like sound-absorbing material, in which rock wool, a glass fiber and a polyester fiber are irregularly oriented in a mixed state and these fibers are bound together with a fibrous binder such as a low-melting point polyester fiber, and a surface material which is comprised of a polyester fiber-based non-woven fabric subjected to water-, oil-, and flame-proof treatment. Further, Japanese Patent Application Laid-open No. 2002-161465 discloses a sound-absorbing material manufactured by laminating a flame-retardant polyester filament non-woven fabric as a surface material on one surface of a laminate structure comprising a meltblown non-woven fabric and a polyester non-woven fabric united by needle-punching.
In both of the above techniques, these flame-retardant sound-absorbing materials are manufactured by uniting a sound-absorbing material with a flame-retardant surface material. According to the former techniques, as described above, since the mat-like sound-absorbing material and the surface material coating the sound-absorbing material are integrally molded, it is necessary to carry out thermocompression molding at a temperature of a melting point of the fibrous binder or higher, which complicates the manufacturing process thereof. Further, in a case where the polyester fiber contains a halogen-based flame retardant, there is a fear that the sound-absorbing material produces a toxic gas when burned. On the other hand, the sound-absorbing materials according to the latter techniques have a drawback that flame retardancy is poor.